Recent reports of inspiratory muscle fatigue in athletes following both high-intensity short-term as well as prolonged endurance exercise have focussed our attention on the question "whether the metabolic O2 requirements of the respiratory muscles are sufficient to limit VO2max and exercise capacity of the locomotor muscles?" As yet only limited fragmented information is available to even begin to answer this very important question. Therefore, we are proposing experiments in healthy miniature swine where blood flow, capillary density, and aerobic metabolic capacity of all respiratory muscles as well as phrenic extraction of O2, lactate and blood-borne substrates, and possible phrenic ammonia production will be studied at rest, and during sub-maximal and maximal exercise performed in normoxia, hypoxia, and hyperoxia without as well as with inspiratory resistive breathing. In the first year the experimentation will be on "sedentary" animals, and these data will serve as control for the second year experiments to be carried out on "exercise-trained" animals. Techniques to be employed in these studies are considered standard, routine, and are widely used; and we also have extensive experience with these procedures. It is believed that proposed research will significantly enhance our knowledge of energetics of respiratory muscles during rest and exercise, and this knowledge may also help improve management of situations where respiratory muscle function may be compromised.